Elliott Sound Products   Project 29

Guitar Tremolo Unit

Rod Elliott (ESP)


Tremolo is one of those simple effects that has just lasted forever (well, almost).  The circuit shown here has wide range, and a very controlled and musical modulation characteristic, and should keep the guitarists happy for minutes at a time.

The project is simple to build, and can even be housed in a pedal if desired.  If the pedal option is used, don't try to run it from batteries, as they will not last very long due to the LED current.  It really needs a +/- 15V supply as shown in the circuit to operate properly.

Tremolo Unit Description

The unit is simple to build, and does not need really low noise opamps, since they only act as a modulator oscillator.  I used 1458 dual types in the prototype, and they are more than good enough.  The transistors can be any low noise NPN type, and they are simply buffers, ensuring a high input impedance and low output impedance.

If the unit is to built into an amplifier, it may well be possible to leave out the input transistor, since a low impedance drive circuit is probably already available from an existing opamp.  It may also be possible to leave out the second transistor if a high impedance input is available at the insertion point.  This is somewhat unlikely, since the most common place to have the modulator is before the tone controls.

Figure 1
Figure 1 - Tremolo Unit Circuit

The opamp power supply pins are:  Pin 4,  -ve and Pin 8, +ve.  This is the same on virtually all dual opamps.  The value of C2 might need to be changed (in some cases it can be omitted) if the load impedance is less than about 20k Ohms.

The oscillator is a simple opamp feedback type, and produces a triangle wave from the capacitor (C3).  This is amplified and buffered, and fed to the LED in the opto-coupler.  If you are unable to obtain this device (made by Vactrol), use a high quality Light Dependent Resistor (LDR) with a LED in a light-proof encapsulation - heat-shrink tubing is good, but you will probably need two layers to ensure it is completely sealed against light getting in.  Use a high output LED, and make sure that the LED and LDR are properly aligned for maximum sensitivity.

The second LDR is used as a panel indicator, and can be any colour you choose.  When switched off, the LEDs will both be off, and the panel LED flashes at the selected rate.  It might be necessary to reduce the value of R10 to ensure that there is enough drive to the LEDs to get the full modulation.

The frequency range is from about 2.5Hz to 14Hz with the values as shown, but this can be changed to suit your needs.  This is generally a good range, and will be more than wide enough for most users.

Amplitude modulation can be varied from none at all, to full modulation with the signal varying from fully on to fully off.  The frequency range that can be covered with full modulation is dependent on the speed of the LDR.  Most of the commonly available ones are fast enough to give a good modulation depth at even the highest frequency.

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Copyright Notice. This article, including but not limited to all text and diagrams, is the intellectual property of Rod Elliott, and is Copyright (c) 1999. Reproduction or re-publication by any means whatsoever, whether electronic, mechanical or electro- mechanical, is strictly prohibited under International Copyright laws. The author (Rod Elliott) grants the reader the right to use this information for personal use only, and further allows that one (1) copy may be made for reference while constructing the project. Commercial use is prohibited without express written authorisation from Rod Elliott.